When I do my Hello Kitty cakes, I just take a round cake, and do a little carving to get the shape of her head, then I just add fondant accents (bow, eyes, nose, etc). It's really not a hard cake to do!! I added a link to my cake below:

There is actually a Hello Kitty Pan available at Party City Stores. But, as aundron says....just a little carving and some eyes, nose, whiskers and bow and you're there. Here is a link to a smash cake I did.